As one of the methods for controlling an internal combustion engine, for example, as disclosed in Japanese Patent Laid-Open No. 2009-047102, a method is known that determines a manipulated variable of each actuator by using efficiency and an air-fuel ratio as well as torque as controlled variables. As used herein, the term “efficiency” refers to a ratio of torque that is actually outputted with respect to a potential torque that the internal combustion engine can output. Further, as used herein, the term “air-fuel ratio” refers to an air-fuel ratio of an air-fuel mixture provided for combustion in the internal combustion engine. There are demands from a variety of viewpoints with respect to the torque, the efficiency, and the air-fuel ratio as controlled variables. In the case of torque, there is torque that is demanded to satisfy an acceleration demand of a driver, and torque that is demanded for driving control, such as skid prevention and the like. In the case of efficiency, there is efficiency that is demanded for warming up a catalyst, and efficiency that is demanded for ensuring reserve torque and the like. In the case of the air-fuel ratio, there is an air-fuel ratio that is demanded in order to restore the NOx reducing ability of a catalyst quickly at the time of reversion from a fuel cut, and an air-fuel ratio that is demanded in order to improve purification performance of a catalyst during operation of the internal combustion engine and the like. According to the control apparatus (hereunder, referred to as “preceding apparatus”) that is described in the aforementioned patent publication, in a case where there are a plurality of demands with respect to a single controlled variable in this manner, the final demanded controlled variable is determined by mediation. As used herein, the term “mediation” refers to a computation process for obtaining a single numerical value from a plurality of numerical values that is performed in accordance with a predetermined rule. As examples of the specific computation rule, selecting the value selection, selecting the minimum value, averaging, or superimposing may be mentioned.
Realization of the final demanded controlled variables obtained by mediation, that is, a demanded torque, a demanded efficiency, and a demanded air-fuel ratio, means realization of various kinds of performance (demanded engine performance) that are demanded of the internal combustion engine, namely, running performance, exhaust gas performance, and fuel consumption performance. According to the preceding apparatus, air amount control, ignition timing control, and fuel injection amount control are executed based on the aforementioned three kinds of demanded controlled variables. In the air amount control, a slot operation is performed in accordance with a target air amount that is calculated based on the demanded torque and demanded efficiency. In the ignition timing control, an operation of an ignition apparatus is performed in accordance with a ratio of a demanded torque with respect to an estimated torque (estimated potential torque) that is calculated based on a degree of throttle opening. Further, in the fuel injection amount control, an operation of a fuel injection apparatus is performed in accordance with a demanded air-fuel ratio. That is, according to the preceding apparatus, an attempt is made to realize each demanded controlled variable by causing three kinds of actuators to co-operate based on three kinds of demanded controlled variables.
However, depending on the rate of change in the demanded controlled variables, it is not necessarily the case that an effect that realizes a demanded engine performance that is the aim of the preceding apparatus can always be achieved. This is because when an operation of an actuator is related to a certain controlled variable, a limit that depends on the kind of the actuator exists with respect to a response speed of the relevant controlled variable to the operation of the actuator. For example, in the case of a throttle, the response speed of an air amount when the throttle is moved at a maximum speed is a limit response speed of the torque with respect to an operation of the throttle. In the preceding apparatus, the throttle is used as the principal actuator for controlling torque, and in a case where a change in any demanded controlled variable is faster than the limit response speed of the torque with respect to an operation of the throttle, a fluctuation in the torque that accompanies that change can not be suppressed by torque control that is performed by means of the throttle. In this respect, if ignition timing control or fuel injection amount control can cause the torque to change more quickly, the aforementioned kind of fluctuation in the torque can be suppressed and the demanded torque can be realized. However, in this case, it is no longer possible to realize, at least temporarily, one of the demanded controlled variables other than the demanded torque, that is, one of the demanded efficiency and the demanded air-fuel ratio. Thus, in this case, it is not possible to realize the demanded engine performance that relates to the demanded efficiency or the demanded air-fuel ratio that can not be realized among the various performances that are demanded of the internal combustion engine.
As described in the foregoing, running performance, exhaust gas performance, and fuel consumption performance are representative of the kinds of performances demanded of an internal combustion engine. However, there is an order of priority between these kinds of performances that depends on the state of the internal combustion engine or the circumstances in which the internal combustion engine is placed. For example, although exhaust gas performance is given priority at startup of the internal combustion engine, running performance is given priority when driving control such as skid prevention is performed. Even if it is not possible to realize all of the demanded engine performances, it is desirable to enable realization of at least the demanded engine performance that has the highest priority. However, according to the above described preceding apparatus, if any demanded controlled variable changes at a fast rate that exceeds a response speed of the relevant controlled variable with respect to an operation of an actuator, even if a demanded engine performance has the highest priority, realization thereof has not been ensured.